1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device suitable for a ferroelectric memory.
2. Description of the Related Art
In recent years, a tendency of processing or storing large-volume data at high speeds has become higher with development of digital techniques. This requires a semiconductor device for use in electronics to be more highly integrated and have a higher performance.
For the semiconductor device, research and development of a technique for achieving a highly integrated DRAM has started, for example. This technique uses a ferroelectric material or a high dielectric constant material for a capacitor-insulation film of a capacitor forming the DRAM, in place of silicon oxide or silicon nitride that is conventionally used.
Moreover, research and development of a technique have also started, which uses a ferroelectric film having spontaneous polarization characteristics as the capacitor-insulation film in order to achieve a nonvolatile RAM for which writing and reading can be performed at a low voltage and at high speeds. This type of semiconductor device is called as a ferroelectric memory (FeRAM).
The ferroelectric memory stores information by using ferroelectric hysteresis. The ferroelectric memory includes a ferroelectric capacitor that is formed with a pair of electrodes and a ferroelectric film sandwiched between the electrodes as a capacitor-dielectric film. In the ferroelectric film, polarization occurs in accordance with a voltage applied across the electrodes and spontaneous polarization is kept even after the applied voltage is removed. When the polarity of the applied voltage is inverted, the polarity of the spontaneous polarization is also inverted. Therefore, information can be read out by detecting the spontaneous polarization. The ferroelectric memory can operate at a lower voltage than a flash memory and allows high-speed writing to be performed while saving a power.
PZT ferroelectric materials, compounds having a Bi-layered structure, and the like are used for the ferroelectric film. As the PZT ferroelectric materials, lead zirconium titanate (PZT) itself, PZT in which La, Ca, Sr, and/or Si is doped, and the like can be used. As the Bi-layered structure compounds, SrBi2Ta2O9 (SBT, Y1) and SrBi2 (Ta, Nb)2O9 (SBTN, YZ) can be used, for example. Those films are formed on a bottom electrode film in an amorphous state by a sol-gel method, sputtering, MOCVD, or the like, and are then crystallized by a heat treatment.
The heat treatment for crystallization is performed by furnace annealing using an annealing furnace or RTA (Rapid Thermal Annealing) using a lamp annealing apparatus. The heat treatment is typically performed at a temperature of 600° C. or higher (e.g., 700° C.) for 60 seconds. In case of furnace annealing, huge crystal grains in which the ferroelectric material is oriented in a direction in which polarization cannot be controlled by an applied voltage may be generated. On the other hand, in case of RTA, generation of such huge crystal grains occurs less easily, as compared with the case of furnace annealing.
Japanese Patent Application Laid-Open No. Hei 11-54716 (Patent document 1) describes annealing in an oxygen atmosphere performed after formation of the PZT film. This annealing is performed by using an RTA apparatus or a resistance furnace at a temperature of 700° C. or higher.
Japanese Patent Application Laid-Open No. 2001-189433 (Patent document 2) describes that, in order to improve shot fail and leak current characteristics, rapid thermal annealing is performed in an oxygen atmosphere at a temperature range of from approximately 700° C. to 800° C. for approximately 20 to 60 seconds after formation of the PZT film, a top electrode is then formed, and thereafter a heat treatment is performed in a furnace in an oxygen atmosphere at a temperature range of from approximately 755° C. to 825° C. for 30 to 90 minutes.
Japanese Patent Application Laid-Open No. 2002-203914 (Patent document 3) describes that annealing is performed twice after the bottom electrode, the PZT film, and the top electrode are formed in order to improve leak current characteristics and dielectric characteristics of the capacitor. The first annealing is performed in an oxygen atmosphere at a temperature range of from 200° C. to 600° C. The second annealing is performed in an oxygen-free atmosphere at a temperature range of from 300° C. to 900° C.
U.S. Pat. No. 6,287,637 (Patent document 4) describes that, when the PZT film is crystallized, annealing is performed twice in order to increase the amount of switching charges and reduce a saturation coercive voltage and fatigue loss. The first annealing is performed in an atmosphere of inert gas containing a small amount of oxygen gas at 600° C. for 90 seconds. The second annealing is performed in an oxygen atmosphere at a temperature range of from 725° C. to 750° C. for 20 seconds.
Japanese Patent Application Laid-Open No. 2001-126955 (Patent document 5) describes that annealing is performed twice in a similar manner to that described in Patent document 4.
However, none of the above conventional methods can sufficiently bring out the characteristics of the ferroelectric film, when the thickness of the ferroelectric film is about 150 nm or less.
[Patent document 1] Japanese Patent Application Laid-Open No. Hei 11-54716
[Patent document 2] Japanese Patent Application Laid-Open No. 2001-189433
[Patent document 3] Japanese Patent Application Laid-Open No. 2002-203914
[Patent document 4] U.S. Pat. No. 6,287,637
[Patent document 5] Japanese Patent Application Laid-Open No. 2001-126955